Research explained

Smoking cannabis increases your chance of a road traffic incident, although no threshold has been defined

By:
Wayne Hall

Published: 20 April 2012

DOI: 10.1136/sbmj.e1735

Cite this as: Student BMJ 2012;20:e1735

For several reasons it is important to know whether driving within several hours of smoking cannabis increases the risk of a motor vehicle collision. Firstly, many young people in Europe, North America, and Australia have used cannabis at some time, a substantial minority do so regularly, and a substantial proportion of regular users drive within several hours of doing so.1 Secondly, young people are at an increased risk of motor vehicle collision because of their relative inexperience and higher levels of risk taking.2 Thirdly, after alcohol, cannabis is the drug that has most often been found in the blood in post-mortem studies of drivers killed in incidents in these countries.3

Two types of study have assessed whether there is a causal relationship between cannabis use and risk of an incident. In laboratory studies researchers randomly assign people to receive cannabis or not and then compare their performance in a driving